1. Field of the Invention
The presently disclosed subject matter relates to a mirror driving device and method of controlling the device and, in particular, relates to the structure of a micromirror device suitable for an optical deflector for use in optical scanning and others and technologies for controlling the driving of the micromirror device.
2. Description of the Related Art
A microscanner (hereinafter referred to as a “MEMS (Micro Electro Mechanical System) scanner fabricated by using silicon (Si) microfabrication processing) has a feature of being small in size and having low power consumption compared with a polygon mirror, which is a conventional optical scanning module, or others. For this reason, the MEMS scanner is expected be widely applied from a laser projector to an optical diagnostic scanner such as an Optical Coherence Tomography (OCT).
Among various schemes for driving the MEMS scanner, a piezoelectric driving scheme using deformation of a piezoelectric substance can achieve a large torque per unit area compared with other schemes and uses a simple drive circuit, and therefore has a potential as a scheme allowing a small size and a large scan angle.
However, a piezoelectric actuator not using resonance has a problem of a small displacement. For example, in the field of endoscopic optical diagnosis such as OCT, the scan length on a target to be measured is desired to be at least 1 mm or more (refer to McCormick, D. et. al., “A Three Dimensional Real-Time MEMS Based Optical Biopsy System for In-Vivo Clinical Imaging” in Solid-State Sensors, Actuators and Microsystems Conference, 2007, TRANSDUCERS 2007, International', pp. 203-208). For example, when a MEMS scanner incorporated in the endoscopic probe having a diameter of 5 millimeters (5 mm φ) performs a radial scan on a point 0.5 mm outside the probe, for a scan of 1 mm or more, an optical deflection angle at least 20 degrees or more is required. Achieving such a large optical deflection angle in a single piezoelectric unimorph cantilever is difficult, and some contrivance is required.
As a means for significantly tilting a mirror, a structure has been suggested in which, for example, a cantilever itself is folded a plurality of times to form a meander shape so as to interpose a mirror part and also a movable plate is used for a gimbal structure, thereby achieving non-resonant two-dimensional (2-D) driving (refer to Tani, M., Akamatsu, M., Yasuda, Y., Toshiyoshi, H., “A two-axis piezoelectric tilting micromirror with a newly developed PZT-meandering actuator” in Micro Electro Mechanical Systems, 2007, MEMS. IEEE 20th International Conference (2007) pp. 699-702). In this scheme, a piezoelectric cantilever is folded to allow driving so as to induce bending alternately in opposite directions, thereby expanding displacement. With this, the mirror can be significantly tilted without using resonance.
However, while this structure can achieve a larger rotation angle as the piezoelectric cantilever is folded more times, resonance frequencies of rotation motion and translational motion of the mirror are decreased at the same time. This causes the following problems.
(1) Resonance of the mirror is easily excited by ambient vibration.
(2) When the mirror is driven for rotation with a triangle wave or a sawtooth wave, resonance vibration (sine wave components) of rotation motion is superposed to change a drive response waveform.
In particular, Problem (1) causes fluctuations of the optical path length and a positional shift of a spot during scanning in the case of a use purpose with a lot of environmental vibrations such as a vehicle-mount purpose or endoscopic purpose, and therefore becomes critical in actual use.
Also, with the adoption of the gimbal structure, the device size is increased, and therefore application for endoscopic purposes generally requiring a device size of 3×3 mm or less is difficult. In view of decreasing the size of the device, a non-resonant two-dimensional (2-D) scanner not adopting a gimbal structure is desired. A non-resonant two-dimensional (2-D) MEMS scanner of a thermal bimorph type not using a gimbal structure was prototyped in Singh, J.; Teo, J. H. S.; Xu, Y.; Premachandran, C. S.; Chen N.; Kotlanka, R.; Olivo, M. & Sheppard, C. J. R. (2008), “A two axes scanning SOI MEMS micromirror for endoscopic bioimaging”, Journal of Micromechanics and Microengineering 18(2), 025001.
However, the structure disclosed in Singh et al., for example, when the mirror is driven for rotation about an x axis, a reaction force works due to an actuator of another axis not involving driving, and therefore drive efficiency is significantly decreased.
By contrast, a structure as depicted in FIG. 26 has been suggested in Japanese Patent Application Laid-Open No. 2008-040240. This FIG. 26 cites the figure disclosed as FIG. 3 in Japanese Patent Application Laid-Open No. 2008-040240. Adjacent actuators as described above are subjected to bending displacements in opposite directions to accumulate deflection angles for transfer to the mirror, and each have a plurality of cantilevers to be subjected to bending rotation about two axial directions orthogonal to each other, thereby allowing two-dimensional rotation about these axes. According to this structure, in the case of rotation about an axis, a reaction force by an actuator responsible for driving about any other axis does not occur, and therefore the force can be effectively used.
However, the structure depicted in FIG. 26 has the following problems.                When extraneous vibration in a vertical direction occurs, an angular displacement occurs to the mirror due to an inertial force.        Since many cantilevers are folded to obtain sufficient displacement, the resonance frequency is low, and the structure is easily influenced by disturbances.        Since the center of the mirror does not match the center of the device, when the MEMS is installed so that the center of the mirror is placed inside a cylindrical tube such as an endoscope, an approximately doubled space is required.        